There is a place called TKO Jeeps out of Texas that was working on a bolt on portal box. I actually saw them on a Jeep at the Off-Road Impact Show last January, but I have not heard much about it since then. I was not very impressed with the product, it just looked really rough to me and the geometry did not look right.
In general I think that portals are great because you can get them for relatively cheap with cable actuated lockers, really low gears, and a ton of ground clearance. They are heavy, but that can be an advantage or disadvantage depending on the situation. The costs start adding up when you look at adding pinion conversions and disc brakes. Uptravel is going to be an issue unless you design the frame around the axles, and even then you have things like the oil pan to deal with. As you mentioned though, this is not such a big deal when you have so much ground clearance. There have been people breaking the cast reduction box housing in competition use, but overall they are very strong.
I think that your biggest challenge is going to be designing a suspension around the portal axles. In addition to the mounting constraints, the reduction boxes act like foot tall lift blocks and leaf springs will not be adequate in the front or back. This may not be a big issue for you though if you are already planning a three or four link suspension.
More Mog portal info can be found at
http://www.exaxt.ca/ and
http://www.killeraxles.com/
